1. Field of the Invention
The invention relates to a quantization process and device for video compression. It is more particularly applied to video coding according to the ISO/IEC MPEG2 standard.
2. Description of the Related Art
Video compression processes carry out a quantization operation in the coding chain. This quantization is generally applied to blocks of coefficients arising from a cosine transform.
The slaving of the quantization interval to the output bit rate from the coder makes it possible in particular to regulate this bit rate. The image quality obtained by a coder at given bit rate is in large part related to this regulating policy. Specifically, one simultaneously requires a quantization interval which is large enough not to exceed the fixed bit rate but small enough to limit the coding artefacts of the block effect type. Moreover, big variations in quantization interval within the image cause defects of the xe2x80x9cbeatingxe2x80x9d type and regulation therefore attempts to stabilize the interval used to the maximum possible extent.
Regulation is based on an objective criterion which is generally the cost of coding the quantized block. If the costs of the previously quantized blocks deviate excessively from the average cost permitted in order to comply with the target bit rate then the quantization interval will increase, in the converse case it will decrease. These variations take no account of the type of the area currently being coded and this may give rise to subjective defects. Specifically, the eye is more or less sensitive to defects depending on the content of the coded sequence. In an area of strong motion the defects will be less visible than in a uniform, stationary area for example.
The aim of the invention is to alleviate the aforesaid drawbacks.
The subject is a quantization process for video data compression comprising a step of calculating a quantization interval relating to a pixel or group of pixels as a function of a desired bit rate, characterized in that it comprises an additional step of modifying the quantization interval as a function of a subjective quality cue relating to this pixel or group of pixels.
In particular, the quality cue is of the JND type (Just Noticeable Difference).
A variant of the invention consists in the cue being averaged in blocks of pixels.
Another variant of the invention consists in the cue relating to a block being filtered in a window around this block.
Another variant of the invention consists in the cue being weighted by a subjective quality cue averaged over the image.
Another variant of the invention consists in the per-pixel subjective quality cue being provided on the basis of a subjective evaluation process generating a degraded image. The degraded image can be the local decoded arising from a previous coding of the corresponding reference image.
Other variants of the invention consist in the degradation of the image being obtained by making the reference image noisy with the aid of Gaussian noise or else by making the reference image noisy with the aid of transform, quantization, inverse transform and inverse quantization operations.
Another variant consists in the quantization interval used for the quantization and inverse quantization operations being that used during the coding of a previous image.
Another variant consists in the quantization interval used for the quantization and inverse quantization operations being that used during a preanalysis of the image.
The invention also relates to a device for implementing the process.